The present invention relates to an image processing system, and more particularly to an image processing apparatus capable of outputting image data supplied from a scanner with a high quality by subjecting supplied image data to various filtering processes and by synthesizing image data subjected to the filtering processes so as to output synthesized image data, and to an image processing method therefor. More particularly, the present invention relates to an image forming apparatus using the image processing apparatus.
In recent years, image forming apparatus, such as digital copying machines and facsimile machines, have been significantly widely used. The digital apparatus of the foregoing type must perform image processes for outputting an image supplied from a scanner and having a high quality. In general, image input/output apparatuses, such as the digital copying machines and facsimile machines, suffer from a problem of deterioration in the quality of an input image occurring attributable to the MTF (Modulation Transfer Function) of the optical system and deterioration in the quality of an input image occurring attributable to generation of return distortion because of limitation of the frequency range required to perform digital sampling. When an image is output, there arises a problem in that the quality of the image deteriorates owning to the developing system and to the spatial frequency characteristic, such as generation of moire peculiar to a digital process.
Therefore, the MTF correction must be performed as a portion of the processes of the image processing system. The foregoing MTF correction includes a filtering process. The filtering process is classified into a low pass filtering process for preventing moire or the like and a high pass filtering process for highlighting edges of characters or the like. In general, the filtering process is realized by a two-dimensional digital filtering process which is performed in the main scanning direction and the subscanning direction. That is, the filtering process is performed by multiplying pixels in a local region with corresponding coefficients, the pixels in the local region being composed of a pixel of interest, which must be processed, and pixels surrounding the pixel of interest. Therefore, when the filtering process is performed in such a manner that the matrix size is (n.times.n), the process in the sub-scanning direction requires a line buffer for n lines.
To perform the low pass filtering process and the high pass filtering process, a structure for reducing the cost and simplifying the structure of the hardware has been suggested in which a line buffer is shared by the low pass filtering process and the high pass filtering process. That is, the line buffer is shared so that the low pass filtering process and the high pass filtering process are performed in parallel to each other. After the low pass filtering process has been ended, image data subjected to the low pass filtering process is subjected to a range correction process. After the high pass filtering process has been ended, a weighting and multiplying process is performed. Image data subjected to the range correction process and image data subjected to the weighting and multiplying process are synthesized, and then synthesized image data is output.
However, the synthesizing process sometimes results in inversion (hereinafter described as a "density inversion phenomenon") of the density between specific image data including edge components and specific image data including no edge component from a macroscopic viewpoint (that is, the density is substantially inverted). The above-mentioned phenomenon becomes conspicuous in proportion to the resolution and the realized level of the gradation of the printer system.
The density inversion phenomenon will briefly be described. Initially, a region from a lowest density level to a highest density level, which are the subjects of the density levels, is uniformly divided into 256 steps (00h to FFh). The lowest density level is made to be 00h and the highest density level is made to be FFh. Then, first image data containing no edge components and second image data containing edge component are considered.
An assumption is made that first image data is a 3.times.3 matrix composed of pixels having a density light of FAh. On the other hand, second image data is a 3.times.3 matrix composed of a pixel of interest having a density level of FAh and pixels surrounding the pixel of interest and each having a density level of FFh (the center of the pixel matrix is the pixel of interest). That is, from a macroscopic viewpoint, the density level of second image data is (substantially) higher than the density level of first image data.
An assumption is made that first image data is subjected to a predetermined low pass filtering process, and then subjected to a predetermined range correction process so that a 3.times.3 pixel matrix composed of density level of FFh is obtained. Another assumption is made that first image data is subjected to a predetermined high pass filtering process, and then subjected to a predetermined multiplying process so that a 3.times.3 pixel matrix having highlighted edges is obtained. Since first image data does not contain any edge component, the obtained 3.times.3 pixel matrix does not include the density level having a minus component. When first image data subjected to the range correction process and second image data subjected to the multiplying process are synthesized, a 3.times.3 pixel matrix composed of the density level FFh is obtained. Therefore, image data is obtained by the synthesizing process which is a 3.times.3 pixel matrix composed of a pixel of interest having the density level of FFh.
An assumption is made that second image data is subjected to a predetermined low pass filtering process, and then subjected to a predetermined range correction process so that a 3.times.3 pixel matrix composed of density level of FFh is obtained. Another assumption is made that second image data is subjected to a predetermined high pass filtering process, and then subjected to a predetermined multiplying process so that a 3.times.3 pixel matrix having highlighted edges is obtained. The density level of the pixel of interest in the 3.times.3 pixel matrix has minus components. When second image data subjected to the range correction process and second image data subjected to the multiplying process are synthesized, the pixel of interest, which must be processed with the density level FFh, is undesirably processed with a density level which is lower than the density level FFh. Therefore, image data is obtained by the synthesizing process which is a 3.times.3 pixel matrix composed of a pixel of interest having the density level of, for example, FAh, and pixels surrounding the pixel of interest and having the density level of FFh.
When the density of first image data and second image data subjected to the synthesizing process are compared with each other, it can be understood that the density level of second image data is, from a macroscopic viewpoint (that is, substantially), lower than the density level of first image data. That is, a fact can be understood that the synthesizing process results in the density level of first image data and the density level of second image data being inverted. The density inversion phenomenon becomes conspicuous in a portion in which the density is moderately changed. The phenomenon is one of causes of deterioration in the quality of the formed image data. Facsimile machines, capable of lowering the resolution by converting the resolution, suffers from a problem in that the local region, in which the density is inverted, is enlarged in the above-mentioned case and thus the deterioration in the quality of the formed image becomes excessive.